=over

=item $a

=item $b
X<$a> X<$b>

Special package variables when using C<sort()>, see L<perlfunc/sort>.
Because of this specialness C<$a> and C<$b> don't need to be declared
(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
pragma.  Don't lexicalize them with C<my $a> or C<my $b> if you want to
be able to use them in the C<sort()> comparison block or function.

=back